Truck & Off-Highway Engineering - October 2021 - 26

Developing a NEXT-GEN VGT
Engineers from Mitsubishi Heavy Industries
refine the design of a variable geometry
turbocharger for commercial vehicles.
V
ariable geometry turbochargers (VGT) have been applied to
commercial engines for a long time, owing to their operability at
wide operation range. One of the major advantages of using a
VGT is its ability to provide high boost pressure at low engine
speeds, which ensures optimum supply of air for proper combustion,
leading to a significant reduction in emissions. Recent emission standards
by U.S. EPA and in Europe (Euro VI) demand a higher efficiency
from the turbine at all operating points, which motivated engineers from
Mitsubishi Heavy Industries to do an in-depth loss analysis of each component
and carry out design modifications to achieve these demands.
The multi-vane VGT, which has been found to be the most effective
among all the configurations, consists of a plurality of nozzle vanes distributed
circumferentially upstream of the radial turbine rotor. These
vanes are controlled by an electric actuator working in coherence with the
engine control unit (ECU) to control the mass flow rate entering the rotor.
There are many different types of link mechanisms to transfer the actuation
force to the vanes; the authors selected a mechanism consisting of a
plurality of lever arms connected to each vane, driven by a drive ring
moved circumferentially using a crank arm connected to the actuator.
This defines a VGT nozzle assembly with pivoted vanes rotating
about an axis parallel to the rotation axis of the turbine rotor. A major
advantage is the wide range of operability with small size and low
cost of production. But this system faces three major challenges during
its operation: the reduction in efficiency due to the leakage flow
from vane side clearance mostly at low mass flow conditions; the
Analysis model for full-scale CFD simulations.
occurrence of mass flow hysteresis during ramp up and
down of the nozzle vanes caused by the resistance of
frictional forces; and the increased risk of damage due
to nozzle wake at resonance frequencies of the rotor.
The development of a new VGT turbine was focused
on achieving better performance for a 4-cylinder 4.0-L
diesel engine for commercial vehicles. As per the engine
low-end torque (LET) and high-end power (HEP)
requirements, the current base model was found to be
off target in terms of efficiency at low engine speeds,
and it involved a conservative design approach to include
only the first vibration mode within limits of
maximum rotation speed. The new requirements of
high-pressure ratio from the compressor at low mass
flow conditions would have made the occurrence of
higher vibration modes inevitable.
The authors took this as an opportunity to design a
Effectiveness of the new design in improving turbine efficiency: (a) contours
showing loss generation at 95% nozzle height plane; (b) streamlines colored by
loss generation at nozzle cross section plane (10% vane opening).
26 October 2021
VGT turbine to achieve a high and stable efficiency
over the entire operation range, with improved vibration
characteristics. The workflow of the tasks carried
out in this research consisted of four steps. First, to
reduce the cost of manufacturing, the size of the VG
nozzle assembly was reduced by increasing the number
of vanes from 10 to 13 and reducing the vane pivot
diameter (PCD) while maintaining the maximum
throat area at 100% nozzle opening. In accordance,
the number of turbine rotor blades was increased
from 8 to 11 to maintain a relationship of 2 nodal diameter
(ND) between stator and rotor. The scallop of
base rotor was removed, and meridional shape was
modified focusing on performance improvement at
low velocity ratio values. Based on the flow analysis,
it was observed that there is further scope of
TRUCK & OFF-HIGHWAY ENGINEERING
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Truck & Off-Highway Engineering - October 2021

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